Digital subscriber line communicating system

ABSTRACT

A digital subscriber line communicating system having a central office and a remote terminal connected through a telephone line, the transmitting side comprising a sliding window transmitting unit for transmitting DMT symbols according to the sliding window through the telephone line to the receiving side, and the receiving side comprising a sliding window receiving unit for receiving DMT symbols according to the sliding window from the transmitting side, the sliding window indicating the phase of cross-talk condition due to a TCM-ISDN transmission at the receiving side, whereby TCM cross-talk can be reduced without largely departing from the standard system.

BACKGROUND OF THE INVENION

1. Field of the Invention

The present invention relates to a digital subscriber line communicatingsystem which utilizes an existing telephone line as a high speed datacommunication line. More particularly, it relates to an improvement of amodulation/demodulation system in a transmission apparatus used in theabove-mentioned transmission system.

In recent years, multimedia services such as internet and so forth haveexpanded through the whole society including usual homes. Accompanied bysuch development, it has been strongly required to promptly provide aneconomical and reliable digital subscriber line communicating system forutilizing such services.

2. Description of the Related Art

(1) An Explanation of an ADSL

As a technique to provide a subscriber line communicating system whichutilizes the existing telephone line as a high speed data communicationline, an XDSL (Digital Subscriber Line) is known. xDSL is acommunicating system which utilizes a telephone line and amodulation/demodulation technique. xDSLs are generally classified into asymmetric type and an asymmetric type. In the symmetric type, upstreamtransmission speed from a subscriber home (hereinafter referred to as aremote terminal side) to an accommodating central office (hereinafterreferred to as a central office side) is symmetric with the transmissionspeed from the central office side to the remote terminal side. In theasymmetric type, the transmission speed from the remote terminal side tothe central office side is asymmetric with the transmission speed fromthe central office side to the remote terminal side.

In the asymmetric xDSLs, there is an Asymmetric DSL (ADSL) modem whichis provided with the G.DMT standard having a downstream transmissionspeed of about 6 Mbit/sec. and the G.lite standard having a downstreamtransmission speed of about 1.5 bits/sec. Both of the G.DMT and G.liteemploy Descrete Multitone (DMT) modulation.

(2) An Explanation of the DMT Modulation

DMT modulation will be explained using G.lite as an example. Thisexplanation and the associated drawing will describe only the downstreammodulation/demodulation from the central office to the remote terminal.However, DMT modulation is also possible in the upstreammodulation/demodulation.

Firstly, transmitting data is input into an ADSL transceiver unit (ATU)in the central office and a non-symbol time (¼ kHz) of the data isstored in a serial to parallel buffer. The stored data are divided intoa plurality of groups. A predetermined number of transmission bits percarrier signal is previously allocated to each group in accordance witha transmitting bitmap which will be described later in detail. Eachgroup is output to an encoder. In the encoder, each group of the inputbit series is converted into a signal point expressed by a complexnumber for an orthogonal amplitude modulation and is output to IFFT. TheIFFT performs the conversion from each of the signal points to transmitthe signal sequences by an inverse fast Fourier transform. The signalsfrom the IFFT are output to a parallel to serial buffer. Here thesixteen points of the outputs of the IFFT are added as a Cyclic Prefixto the head of each DMT symbol. The output of the parallel to serialbuffer is supplied to a D/A converter in which the digital signal with asampling frequency of 1.104 MHz is converted into an analog signal. Theanalog signal is transmitted through a metalic line to a remoteterminal.

At the remote terminal side, the analog signal is converted into adigital signal with the sampling frequency of 1.104 MHz by an A/Dconverter. Each DMT symbol of the digital signal is stored in a serialto parallel buffer. In the buffer, the Cyclic Prefix is removed from thedigital signal, and the remaining signal is output to an FFT. In theFFT, a fast Fourier transform is effected to generate or demodulate thesignal points. The demodulated signal points are decoded by a decoder inaccordance with a receiving bitmap having the same values as those inthe transmitting bitmap. The decoded data are stored in a parallel toserial buffer as receiving data of bit-sequences.

(3) A Detailed Explanation of the Bitmap

The bitmap described in the explanation of the DMT will be explained indetail with reference to FIGS. 13A and 13B.

The apparatus at the central office side and the apparatus at the remoteterminal side both measure the ratio of the receiving signal to noise(hereinafter referred to as S/N) during a training period prior tocommunication to determine the number of bits to be transmitted by eachmodulating carrier. As shown in FIGS. 13A and 13B, for a carrier signalwith a larger S/N, a larger number of bits to be transmitted areallocated; and for a carrier signal with a smaller S/N, a smaller numberof bits to be transmitted are allocated.

By the above allocation, the receiving side measures the S/N to preparethe bitmap which indicates the numbers of bits to be transmittedcorresponding to the carrier numbers.

The receiving side informs this bitmap to the transmitting side during atraining period so that both the transmitting side and the receivingside can perform the modulation/demodulation with the use of the samebitmap during normal data communication.

(4) Countermeasure Against Cross-Talk From The Time CompressionModulation ISDN Transmission (hereinafter referred to as TCM ISDNTransmission)

When there is a cross-talk due to the TCM ISDN Transmission, in theprior art, two different bitmaps are used in the ADSL modem in thetransmitting side or in the receiving side so as to improve thetransmission characteristic. This method of using the two bitmaps willbe explained with reference to FIG. 14.

In the TCM ISDN transmission, the central office side transmitsdownstream data during a prior half of one cycle of a reference clocksignal of 400 Hz shown in (1) of FIG. 14, in synchronization with thereference clock signal of 400 Hz; and the remote terminal side receivesthe downstream data and then transmits upstream data. Therefore, theADSL modem in the central office is influenced by a Near End Cross-Talk(hereinafter referred to as NEXT) from the ISDN during the prior half ofthe one cycle of 400 Hz, and is influenced by a Far End Cross-Talk(hereinafter referred to as FEXT) from the upstream data of the remoteterminal side ISDN.

Contrary to the central office, the ADSL modem in the remote terminal isinfluenced by a FEXT during a prior half of one cycle of the referenceclock signal of 400 Hz, and is influenced by a NEXT during a latter halfof the cycle.

If the metalic cable between the central office and the remote terminalis long, the S/N of the receiving signal to the NEXT is made smaller,and in some cases, the NEXT may be greater than the receiving signal.

In these cases, since the influence of the FEXT is not so large, in theprior art, two bitmaps are provided. One is a bitmap (DMT symbol X) forreceiving signals during the NEXT period at the remote terminal. Theother is a bitmap (DMT symbol Y) for receiving signals during the FEXTperiod at the remote terminal. During the NEXT period, in the prior art,the number of bits to be transmitted is made small so as to improve theresistance of the signals against the S/N. During the FEXT period, inthe prior art, the number of bits to be transmitted is made large so asto increase the transmission capacity.

On the other hand, the time interval of one DMT symbol is usually 246 μswith a Cyclic Prefix of 16 points. Contrary to this, in the prior art,in order to conform the one DMT symbol with the TCM Cross-talk period of400 Hz, the time interval of one DMT symbol is made to be 250 μs with aCyclic Prefix of 20 points so that one period of the TCM Cross-talk ismade to coincide with the time period of ten DMT symbols, whereby thesynchronization with the TCM Cross-talk is established.

The above-mentioned prior art method of employing the two bitmaps,however, is largely different from the standard system in which only asingle bitmap is employed. If two bitmaps are employed, the sequence ofinforming the bitmaps obtained from the S/N during a training periodfrom the receiving side to the transmitting side must be modified, andin addition, the informing period is doubled so that the training periodis increased.

In the apparatus of the central office or the remote terminal, thememory capacity must be increased in order to store the bitmaps, so thata cost problem occurs.

Further, to change the length of the Cyclic Prefix is largely differentfrom the specification of the standard system so that theabove-mentioned countermeasure against the TCM cross-talk cannot beperformed in the hardware of the apparatus employing the standardsystem.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a digital subscriberline transmission system capable of performing a countermeasure againstTCM cross-talk, without largely departing from the standard system butby modifying only a part of the hardware under the standard system.

Another object of the present invention is to provide a digitalsubscriber line transmission system which can communicate at a mostsuitable transmission speed, regardless of whether or not there iscross-talk.

To attain the above objects, there is provided, according to the presentinvention, a digital subscriber line communicating system forcommunicating between a transmitting side and a receiving side through acommunication line, comprising: a sliding window generating unit forgenerating a sliding window based on a timing signal representing aperiodical noise duration; and a sliding window transmitting unit fortransmitting. modulated symbol according to the sliding window throughthe communication line to the receiving side.

The periodical noise duration is caused with a cross-talk noise on thecommunication line from an another transmission system using timecompression modulation.

Both the sliding window generating unit and the sliding windowtransmitting unit are located in the transmitting side.

The sliding window is generated in such a way that inside modulatedsymbol of the sliding window is received by the receiving side when thereceiving side is in a far end cross-talk duration.

According to an aspect of the present invention, the transmission sideis a central office and the receiving side is a remote terminal. In thisaspect, the central office comprises: a timing signal generating unitfor generating the timing signal synchronized with the transmissionsystem which interferes the central office and the remote terminal. Thesliding window generating unit is operatively connected to the timingsignal generating unit, and the sliding window is a downstream slidingwindow indicating the phase of noise condition of the remote terminal.Also, the sliding window transmitting unit transmits modulated symbolsaccording to the downstream sliding window through the communicationline to the remote terminal. Further, the remote terminal comprises: asliding window receiving unit for receiving modulated symbols accordingto the downstream sliding window from the central office. The downstreamsliding window indicates cross-talk durations due to the TCM ISDNtransmission at the remote terminal.

The downstream sliding window is generated in such a way that insidesymbol of the downstream sliding window is received by the remoteterminal in a first cross-talk duration determined with a far endcross-talk duration at the remote terminal.

The first cross-talk duration is within a prior half of each cycle ofthe timing signal, and a second cross-talk determined with a near endcross-talk duration at the remote terminal, is within a latter half ofeach cycle of the timing signal,

Inside of the downstream sliding window is formed within the firstcross-talk duration.

During timing recover training between the central office and the remoteterminal, inside symbol of the downstream sliding window is formed by afirst kind of signal, and outside symbol of the downstream slidingwindow is formed by a second kind of signal, the first kind of signaland the second kind of signal being obtained by modulating a carriersignal but being different in phase by a predetermined angle.

When the first modulated symbol is synchronized with the head of onecycle of the timing signal, the central office comprises a durationdiscriminating unit for discriminating whether N-th modulated symbolbelongs to inside or outside of the downstream sliding window.

The central office includes a transceiver comprising the timing signalgenerating unit and the sliding window generating unit.

According to another aspect of the present invention, the transmissionside is a remote terminal and the receiving side is a central office. Inthis aspect, the remote terminal comprises: a timing signal receivingunit for receiving a timing phase via received modulated symbolaccording to a downstream sliding window from the central office, thetiming signal being synchronized with a transmission system whichinterferes the central office and the remote terminal. In this aspectalso, the sliding-window generating-unit is operatively connected to thetiming signal receiving unit, and the sliding window is an upstreamsliding window indicating the phase of noise condition of the centraloffice; and a sliding window transmitting unit for transmits modulatedsymbols according to the upstream sliding window through thecommunication line to the central office. The upstream sliding windowindicates a cross-talk duration due to the TCM ISDN transmission at thecentral office. The upstream sliding window is generated in such a waythat an inside symbol of the upstream sliding window is received by thecentral office in a third cross-talk duration determined with a far endcross-talk duration at the central office. In an embodiment, a fourthcross-talk duration determined with a near end cross-talk duration atthe central office is within a prior half of each cycle of the timingsignal, and the third cross-talk duration is within a latter half ofeach of the timing signal, and inside of the upstream sliding window isformed within the third cross-talk duration. When the first modulatedsymbol is synchronized with the head of one cycle of the timing signal,the remote terminal comprises a duration discriminating unit fordiscriminating whether N-th modulated symbol belongs to inside oroutside of the upstream sliding window.

During training between the transmitting side and the receiving side, atraining sequence switching symbol is transmitted from the transmittingside in such a way that the receiving side receives the head of thetraining sequence switching symbol during a far end cross-talk duration.

The number of bits to be transmitted per a carrier signal corresponds toa signal to noise ratio for the carrier signal, only the modulatedsymbols received completely inside of a near end cross-talk duration atthe receiving side being used to measure the NEXT duration S/N, and onlythe inside modulated symbols of the sliding window at the receiving sidebeing used to measure the FEXT duration S/N.

The digital subscriber line communicating system further comprises asliding window bitmap transmission system for transmitting data symbolsonly inside of the sliding window with transmitting capacity determinedby the S/N measurement in the inside of the sliding window at thereceiving side.

The digital subscriber line communicating system further comprises astandard transmission system, wherein, according to the standardtransmission system, data symbols are transmitted in both inside andoutside of the sliding window with transmitting capacity determined bythe S/N measurement in NEXT duration at the receiving side; and whereinthe system having the larger transmitting capacity is selected toperform the communication.

The digital subscriber line communicating system comprises modifiedsliding window bitmap transmission system for transmitting data symbolsin both inside and outside of the sliding window, and the inside datasymbols are transmitted with transmitting capacity determined by the S/Nmeasurement in the inside of the sliding window and the outside datasymbols are transmitted with transmitting capacity determined by the S/Nmeasurement in the NEXT duration at the receiving side.

According to one of the sliding window bitmap transmission system, atleast a pilot tone used for synchronization of timing is transmittedoutside of the sliding window.

According to one of the sliding window bitmap transmission system andthe modified sliding window bitmap transmission system, a firstpredetermined number of super frames, each of which is composed ofsecond predetermined number of modulated symbols and a synchronizingsymbol, constitute a single unit, the single unit being synchronizedwith an integer multiple of one cycle duration of the timing signal, andone of the synchronizing symbols in the single unit, i.e., an inversesynchronizing symbol, is made different from other the synchronizingsymbol in order to maintain the single unit to be synchronized betweenthe central office and the remote terminal, and the inversesynchronizing symbol in N-th super frame of the super frames is receivedin the FEXT duration at the receiving side.

In an embodiment, the N-th super frame is 4-th super frame fordownstream and first super frame for upstream, and the firstpredetermined number of super frames is 5, the second predeterminednumber of modulated symbols is 68.

According to further aspect of the present invention, there is provideda transceiver in a central office connected through a communication lineto a remote terminal, the transceiver comprising: a timing signalgenerating unit for generating the timing signal representing aperiodical noise duration; a sliding window generating unit, operativelyconnected to the timing signal generating unit, for generating adownstream sliding window indicating the phase of noise condition of theremote terminal; and a sliding window transmitting unit for transmittingmodulated symbols according to the downstream sliding window through thecommunication line to the remote terminal.

The periodical noise duration is caused with a cross-talk noise on thecommunication line from an another transmission system using timecompression modulation.

The downstream sliding window is generated in such a way that an insidesymbol of the downstream sliding window is received by the remoteterminal in a far end cross-talk duration at the remote terminal i.e.,R-FEXT duration.

The first cross-talk duration is within a prior half of each cycle ofthe timing signal, and a second cross-talk duration determined with anear end cross-talk duration at the remote terminal is within a latterhalf of each cycle of the timing signal, inside of the downstreamsliding window being formed within the first cross-talk duration.

During timing recover training between the central office and the remoteterminal, inside symbol of the downstream sliding window is formed by afirst kind of signal, and outside symbol of the downstream slidingwindow is formed by a second kind of signal, the first kind of signaland the second kind of signal being obtained by modulating a carriersignal but being different in phase by a predetermined angle.

When the first modulated symbol is synchronized with the head of onecycle of the timing signal, the central office comprises a durationdiscriminating unit for discriminating whether N-th modulated symbolbelongs to inside or outside of the downstream sliding window.

According to still further aspect of the present invention, there isprovided a transceiver in a remote terminal connected through acommunication line to a central office, the transceiver comprising: atiming signal receiving unit for receiving a timing phase via receivedmodulated symbol according to a downstream sliding window from thecentral office, the timing signal being synchronized with a transmissionsystem using time compression modulation which interferes the centraloffice and the remote terminal; a sliding window generating unit,operatively connected to the timing signal receiving unit, forgenerating an upstream sliding window indicating the phase of noisecondition of the central office; and a sliding window transmitting unitfor transmitting modulated symbols according to the upstream slidingwindow through the communication line to the central office; theupstream sliding window indicating cross-talk duration due to the TCMISDN transmission at the central office.

The upstream sliding window is generated in such a way that insidesymbol of the upstream sliding window is received by the central officein a far end cross-talk duration at the central office i.e., C-FEXTduration.

A near end cross-talk duration at the central office, i.e., C-NEXTduration, is within a prior half of each cycle of the timing signal, andthe third cross-talk duration is within a latter half of each of thetiming signal, inside of the upstream sliding window being formed withinthe third cross-talk duration.

When the first modulated symbol is synchronized with the head of onecycle of the timing signal, the remote terminal comprises a durationdiscriminating unit for discriminating whether N-th modulated symbolbelongs to inside or outside of the upstream sliding window.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and features of the present invention will be moreapparent from the following description of the preferred embodimentswith reference to the accompanying drawings, wherein:

FIG. 1A is a block diagram showing a central office according to anembodiment of the present invention;

FIG. 1B is a block diagram showing a remote terminal according to anembodiment of the present invention;

FIG. 2, is a diagram showing a method for transmitting synchronizationsignals according to an embodiment of the present invention;

FIG. 3 is a diagram showing how to define noise durations during aninitial training period according to an embodiment of the presentinvention;

FIG. 4 is a timing chart of transmitting a sequence switching symbolaccording to an embodiment of the present invention;

FIG. 5 is a diagram showing how to define noise durations in receivingsymbols during measuring an S/N, according to an embodiment of thepresent invention; FIG. 6 is a block diagram of an embodiment formeasuring the S/N in each of the NEXT or FEXT duration, according to anembodiment of the present invention;

FIG. 7 is a graph showing transmitting capacities according to thestandard method and the SWB method of the present invention;

FIG. 8 is a diagram showing bitmaps according to the standard method andthe SWB method;

FIG. 9 is a diagram showing a transmission pattern from the centraloffice according to the SWB method;

FIG. 10 is a diagram showing a transmission pattern from the remoteterminal according to the SWB method;

FIG. 11 is a diagram showing how to define the noise durations duringcommunication;

FIG. 12 is a diagram showing the SWB method when two bitmaps areemployed;

FIGS. 13A and 13B show how to define the diagram showing number of bitsto be transmitted to obtain a bitmap;

FIG. 14 is a diagram showing a prior art; and

FIG. 15 is a diagram showing a transmitting pattern of each DMT symbolto inform the phase of the reference clock during timing recoverytraining sequence.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention will be described in thefollowing with reference to the drawings.

FIG. 1A is a block diagram showing functional blocks of a central officeaccording to an embodiment of the present invention; and FIG. 1B is ablock diagram showing functional blocks of a remote terminal accordingto an embodiment of the present invention.

As shown in FIG. 1A, the central office includes a reference clocksignal generating unit 1, a sliding window generating unit 2, and asliding window transmitting unit 3. The reference signal generating unit1 generates a reference clock signal having a frequency of, for example,400 Hz, synchronized with a TCM ISDN transmission which interferes thecentral office and the remote terminal. The reference clock signal maybe generated based on an external signal or within an internal signalgenerated by a crystal oscillator as an example.

The sliding window generating unit 2 generates a downstream slidingwindow from the generated reference clock signal. The downstream slidingwindow discriminates whether the transmitting DMT symbols are receivedin a far end cross-talk duration or in the other duration at the remoteterminal.

The sliding window transmitting unit 3 transmits the DMT symbolsaccording to the downstream sliding window to the remote terminal.

As shown in FIG. 1B, the remote terminal includes a sliding windowreceiving unit 4, a reference clock signal generating unit 5, and asliding window generating unit 6.

The sliding window receiving unit 4 receives the DMT symbol according tothe downstream sliding window from the central office.

The reference clock signal generating unit 5 generates a reference clocksignal based on the reference clock signal generated by the referenceclock signal generating unit 1 in the central office, and transmittedfrom the central office to the remote terminal.

The sliding window generating unit 6 generates an upstream slidingwindow from the generated reference clock signal by the reference clocksignal generating unit 5. The generated downstream sliding windowdiscriminates whether the received DMT symbols are received in a far endcross-talk duration or other duration at the remote terminal.

The reference clock signal in the central office or in the remoteterminal may be generally referred to as a timing signal which issynchronized with the transmission system which interferes the centraloffice and the remote terminal.

The DMT modulation will be explained using the G.lite as an example,with reference to FIGS. 1A and 1B. This explanation and the associateddrawing will describe only the downstream modulation/demodulation fromthe central office to the remote terminal. However, the DMT modulationis also possible in the upstream modulation/demodulation.

Firstly, transmitting data is input into an ADSL transceiver unit (ATU)in the central office and a non-symbol time (¼ kHz) of the data isstored in a serial to parallel buffer 10. The stored data are dividedinto a plurality of groups. A predetermined number of transmission bitsb0, . . . , or bi per a carrier signal is previously allocated to eachgroup in accordance with a transmitting bitmap 60 which will bedescribed later in detail. Each group is output to an encoder 20. In theencoder 20, each group of the input bit series is converted into asignal point expressed by a complex number for an orthogonal amplitudemodulation and is output to IFFT 30. The IFFT 30 performs the conversionfrom each of the signal points to transmit signal sequence by an inversefast Fourier transform. The signals from the IFFT 30 are output to aparallel to serial buffer 40. Here the sixteen tail points 240-255 ofthe outputs of the IFFT 30 are added as a Cyclic Prefix to the head ofeach DMT symbol. The output of the parallel to serial buffer 40 issupplied to a D/A converter 50 in which the digital signal with asampling frequency of 1.104 MHz is converted into an analog signal. Theanalog signal is transmitted through a metalic line 100 to a remoteterminal.

At the remote terminal side, the analog signal is converted into adigital signal with the sampling frequency of 1.104 MHz by an A/Dconverter 110. Each DMT symbol of the digital signal is stored in aserial to parallel buffer 120. In the buffer 120, the Cyclic Prefix isremoved from the digital signal, and the remaining signal is output toan FFT 130. In the FFT 130, a fast Fourier transform is effected togenerate or demodulate the signal points. The demodulated signal pointsare decoded by a decoder 140 in accordance with a receiving bitmap 160having the same values as those in the transmitting bitmap 60. Thedecoded data are stored in a parallel to serial buffer 150 as receivingdata of bit-sequences b0, and bi.

FIG. 2 is a diagram showing a method for transmitting synchronizationsignals according to an embodiment of the present invention. In FIG. 2,(1) represents a reference clock signal for transmitting a TimeCompression Modulation (TCM) signal having a frequency of 400 Hz betweenthe central office to the remote terminal; (2) represents NEXTdurations, i.e., C-NEXT durations and FEXT durations, i.e., C-FEXTdurations at the central office which are synchronized with thereference clock signal (1) of 400 Hz; (3) represents FEXT durations,i.e., R-NEXT durations and NEXT durations, i.e., R-NEXT durations at theremote terminal which are synchronized with the reference clock signal(1) of 400 Hz; (4) represents symbols A and B transmitted from thecentral office to the remote terminal during an initial training; and(5) represents symbols A and B received by the remote terminal. The timedifference between (4) and (5) is the propagation delay. Each of thesymbols A and B has a duration of 256 samples. The symbols A and B areused to inform the NEXT duration and the FEXT duration from the centraloffice to the remote terminal.

The symbols A and B are signals obtained by selecting a carrier having arelatively low frequency at which TCM cross-talk is small; modulatingthe selected carrier by the 4-value QAM as an example to obtain 4 signalpoints; and selecting two signal points A and B from the 4 signalpoints. The phases of the two signal points A and B are different by 90°to each other. The selected signal points A and B are shown in FIG. 15,as an example. The two signal points are converted by the IFFT 30 fromthe frequency domain to the time domain.

At the remote terminal, it is impossible to discriminate each boundaryof DMT symbols output from the IFFT 30 in the central office. Therefore,it is impossible to coincide an FFT interval with a DMT symbol interval,so that signal points after modulation do not appear in correct phasesor quadrants. However, by employing the two symbols A and B havingphases different by 90° to each other, the modulated two symbols alsohave phases different by 90° to each other so that, even when there isan error in phase of the modulated signal points, the remote terminalcan discriminate its NEXT duration, i.e., R-NEXT duration from its FEXTduration, i.e., R-FEXT duration.

FIG. 3 shows how to define the R-NEXT duration and the R-FEXT durationwhen a signal having a frequency of 400 Hz is transmitted during aninitial training.

At the central office, once an ADSL modem detects the phase of thereference clock signal having the frequency of 400 Hz, a DMT symbolcounter for counting each sample and a counter for discriminating theNEXT duration and the FEXT duration at the central office, i.e., theC-NEXT duration and the C-FEXT duration, are started, whereby it becomespossible to discriminate whether a received DMT symbol belongs to theC-NEXT duration or the C-FEXT duration without generating the referenceclock signal from the received DMT symbols.

In FIG. 3, (1) shows counter values “a”, “b”, and “c”, The counter value“a” represents the FEXT duration at the remote terminal; the countervalue “b” represents the NEXT duration at the remote terminal; and thecounter value “c” represents the remaining period obtained bysubtracting (a+b) from one cycle period of the reference clock signal.These values are determined by taking a round trip delay generated by apropagation delay in the TCM ISDN transmission, (2) in FIG. 3 shows thecase when all of the received DMT symbols are included in the R-FEXTduration at the remote terminal; and (3) in FIG. 3 shows the case when apart of the received DMT symbols are included in the R-NEXT duration atthe remote terminal.

When the first DMT symbol is synchronized with the head of one cycle ofthe reference clock signal of 400 Hz as shown in (2) of FIG. 3, thedetermination of whether n-th DMT symbol belongs to the R-FEXT durationor the R-NEXT duration can be performed as follows.

It is assumed that there are 2760 samples in one cycle of the referenceclock signal of 400 Hz, as shown in (1) of FIG. 3. Also, each symbol isassumed to have 256 samples during training as shown in (2) of FIG. 3.Then, a parameter S is defined as:S={256*(n−1)}mod2760.

If {(S<(a−256)} or {S>(a+b)} is satisfied, then it is judged that then-th symbol belongs to an R-FEXT duration.

If {(a−256)≦S≦(a+b)} is satisfied, then it is judged that the n-thsymbol belongs to an R-NEXT duration.

From an ADSL modem in the central office, a sequence switching symbol istransmitted to inform the switching timing of the training sequence tothe opposite party. If the receiving side cannot recognize the head ofthe sequence switching symbol, it is impossible to normally perform thetraining. In order to surely inform the sequence switching, the sequenceswitching symbol is transmitted at a time when the receiving side canreceive the head of the sequence switching symbol during a FEXT durationaccording to an embodiment of the present invention at the receivingside.

FIG. 4 shows the timing of the sequence switching symbol informed fromthe central office to the remote terminal. In FIG. 4, (1) represents thereference clock signal of 400 Hz; (2) shows the C-NEXT durations andC-FEXT durations at the central office; (3) shows the head of thesequence switching symbol transmitted from the central office; (4) showsthe head of the sequence switching symbol received by the remoteterminal; and (5) shows the R-FEXT durations and R-NEXT durations at theremote terminal. The slashed portions in the figure represent the headof the sequence switching symbol. As shown in (3) and (4) of FIG. 4, thehead of the sequence switching symbol is received during the R-FEXTduration at the remote terminal.

In the ADSL modem also, during training, a signal to noise S/N ismeasured for each modulating carrier in the receiving signal todetermine the number of bits to be transmitted for each modulatingcarrier. Under the TCM cross-talk environment, the S/N measurement mustbe performed in each of the NEXT durations and the FEXT durations bytaking the influence of the NEXT or the FEXT into account.

FIG. 5 shows how to define the NEXT duration and the FEXT duration formeasuring the S/N. In FIG. 5, (1) shows the reference clock signal of400 Hz; (2) shows the original R-FEXT duration and the original R-NEXTduration at the remote terminal when S/N is not measured; (3) shows thedefinition of an R-FEXT duration “a” for measuring S/N and of an R-NEXTduration “e” for measuring S/N; (4) shows DMT symbols in the FEXTduration“a”; and (5) shows DMT symbols in the R-NEXT duration “e”. Asshown in FIG. 5, the R-NEXT duration “a” for measuring S/N and theR-FEXT duration “e” for measuring S/N are defined within the originalR-FEXT duration and the original R-NEXT duration, respectively. Thenumber of bit calculated from S/N measured in the NEXT duration must bea value which can ensure a predetermined bit error rate (hereinafterreferred to as BER). To this end, as shown in (4) of FIG. 5, only theDMT symbols within the R-FEXT duration “a” are used to measure the S/Nin the R-FEXT duration; and as shown in (5) of FIG. 5, only the DMTsymbols within the-R-NEXT duration “e” are used to measure the S/N inthe R-NEXT duration. The DMT symbols which are not included in eitherthe R-FEXT duration “a” or R-NEXT duration “e” are not used to measurethe S/N because they have no meaning as information to determine thenumber of bits to be transmitted.

When the first symbol of the DMT symbols is synchronized with the headof the cycle of the receiving signal of 400 Hz, the determination ofwhether the n-th symbol belongs to the FEXT duration for measuring S/Nor the NEXT duration for measuring S/N can be performed as follows.

It is assumed that there are 2760 samples in one cycle of the referencesignal of 400 Hz, as shown in (1) of FIG. 5. Also, each symbol isassumed to have 272 samples during communication, as shown in (4) ofFIG. 5. Then, a parameter S is defined as:S={272*(n−1)}mod2760.

If {(S<(a−272)} or {S>(a+d+e+f)} is satisfied, then it is judged thatthe n-th symbol belongs to an R-FEXT duration for measuring S/N.

If {(a+d)<S<(a+d+e−272)} is satisfied, then it is judged that the n-thsymbol belongs to an R-NEXT duration for measuring S/N.

If any one of the above conditions is not satisfied, then the n-thsymbol is not considered for measuring S/N.

If should be noted that (d+e+f) is equal to “b”in FIG. 3 or in FIG. 11.

FIG. 6 is a block diagram of an S/N measuring unit in the ADSL modem inthe remote terminal.

When a demodulator 210 receives receiving data, it outputs signal pointsof each carrier signal as demodulated data. A reference unit 220 outputssignal points of respective carrier signals which are to be receivedwhen there is no error. The difference between a signal point from thereference unit 220 and a corresponding demodulated signal point from thedemodulator 210 is an ERROR. The ERROR is input to a selector 260.

Further, a clock signal generated from a clock generator 230 in theremote terminal is divided by a frequency divider 240 into a signalhaving a frequency of 400 Hz. The phase of the signal of 400 Hzgenerated by the frequency divider 240 is synchronized with the phase ofthe signal of 400 Hz transmitted from the central office. The signal of400 Hz from the frequency divider 240 is input to a phase discriminator250. The phase discriminator 250 judges, based on the signal of 400 Hzinput into the phase discriminator 250, that the received DMT symbolbelongs to a FEXT duration, a NEXT duration, or other duration. Thejudged result is input to a selector 260. The selector 260 transfers theabove-mentioned ERROR to a NEXT duration S/N measuring unit 270 or aFEXT duration S/N measuring unit 280, in accordance with the judgedresult from the phase discriminator 250. Each of the S/N measuring unitsintegrates the ERRORs to calculate S/N. The S/N for each carrier signalis output to a transmitting capacity calculating unit 290. Thetransmitting capacity calculating unit 290 calculates the number of bitsto be transmitted for each carrier signal, based on the S/N of eachcarrier signal, to output a bitmap b-NEXT for a NEXT duration and abitmap b-FEXT for a FEXT duration.

The ADSL modem in the remote terminal calculates a transmitting capacitybased on the b-NEXT and the b-FEXT. That is, based on the fact that thevalue in the b-FEXT duration is the number of bits to be transmittedwhich can be received during R-FEXT durations only, and the value in theb-NEXT duration is the number of bits to be transmitted which can bereceived in all durations, the following two values are obtained:a transmitting capacity 1=(b−total bit number in FEXT)×α×modulationrate; anda transmitting capacity 2=(b−total bit number in NEXT)×1.0 ×modulationrate.

Then the larger transmitting capacity is selected by communicationbetween the central office and the remote terminal.

Here the method to transmit data in all durations by using the bitmapb-NEXT is referred to as the standard method; and the method to transmitdata only during R-FEXT durations is referred to as sliding windowbitmap (hereinafter referred to as SWB) method.

FIG. 7 is a graph showing the transmitting capacity in the standardmethod and in the SWB method. The solid curve in the figure representthe standard method; and the dashed curve represents the SWB method. Asshown in FIG. 7, under an environment where there is a TCM cross-talk,when the standard method is employed, the longer the length of the linebecomes, the larger the influence of the NEXT; in contrast, when the SWBmethod is employed, even though the transmitting capacity is not highwhen the line is short, the transmitting capacity is not largely loweredeven when the length of the line becomes large.

When the line length is L, the transmitting capacity according to thestandard method is the same as the transmitting capacity according tothe SWB method. Therefore, it is preferable to select the standardmethod when the line length is shorter than the length L, and to selectthe SWB method when the line length is longer than the length L.

FIG. 8 shows a transmitting DMT symbols according to the standard methodand the SWB method. In FIG. 8, (1) shows the reference signal of 400 Hz;(2) shows the NEXT durations and FEXT durations at the central office;(3) shows DMT symbols transmitted from the central office according tothe standard method; (4) shows DTM symbols X obtained by the b-NEXTbitmap; and (5) shows DMT symbols Y obtained by the b-FEXT bitmap.

According to the SWB method, the transmitting side slides the window soas to allocate transmitting bits to each carrier signal only when thetransmitting side is in the C-NEXT durations, that is, only when thereceiving side is in the R-FEXT durations, and the receiving side slidesthe window to demodulate the received data during the R-FEXT durations,as shown in (5) of FIG. 8.

Further, the transmitting signal of a DMT symbol outside the slidingwindow may be a pilot tone for a timing synchronization, and the othercarrier signal may be any signal.

FIG. 9 shows a transmitting signal pattern transmitted from the centraloffice according to the SWB method.

In FIG. 9, (1) shows the reference clock signal of 400 Hz; (2) shows theFEXT durations and the NEXT durations at the remote terminal; and (3)shows the transmitting signal pattern transmitted from the centraloffice.

The ADSL modem in the central office generates one super frame by 69 DMTsymbols. In the 69-th DMT symbol, a synchronizing symbol S indicatingthe boundary of the super frame is inserted. The synchronizing symbol Sdoes not include user data. The ADSL modem transmits the above-mentionedsuper frames.

According to the SWB method, five super frames form a single unit. Thetime duration of the single unit is made to coincide with an integermultiple of the time duration (2.5 ms) of one cycle of the referenceclock signal of 400 Hz shown in (1). In order to allow the remoteterminal to recognize the fifth super frame as a boundary of the superframes, the fourth synchronizing symbol S is inverted in the centraloffice to be an inverted synchronizing symbol I. Thus the signal pointof the inverted synchronizing signal I is different by 180° from thesignal point of the synchronizing signal S. By sending the invertedsynchronizing signal I in the position of the fourth synchronizingsymbol, the remote terminal can receive this inverted synchronizingsignal I in an R-FEXT duration so that the remote terminal can surelyestablish a synchronization of its own SWB with the SWB of the centraloffice.

FIG. 10 shows a transmitting signal pattern transmitted from the remoteterminal according to the SWB method.

In FIG. 10, (1) shows the reference signal of 400 Hz; (2) shows the NEXTdurations and the FEXT durations at the central office; and (3) showsthe transmitting signal pattern transmitted from the remote terminal.

The transmitting signal pattern transmitted from the remote terminal issimilar to that transmitted from the central office. That is, slidingwindows are formed to allow the central office to receive signals duringits FEXT durations. Similar to the central office, the remote terminalalso generate a single unit consisting of five super frames. In order toallow the central office to recognize the boundary of the five superframes, the first synchronizing symbol is inverted in the remoteterminal to be an inverted synchronizing symbol I. Thus the signal pointof the inverted synchronizing signal I is different by 180° from thesignal point of the synchronizing signal S. By sending the invertedsynchronizing signal I in the position of the first synchronizingsymbol, the central office can receive this inverted synchronizingsignal I in a FEXT duration so that the central office can detect thatthe remote terminal is correctly in synchronization according to the SWBmethod.

FIG. 11 shows how to define the R-NEXT duration and the R-FEXT durationwhen a signal of 400 Hz is transmitted during data communication.

During data communication, when all of the samples in a DMT symbol otherthan the Cyclic Prefix are within an FEXT duration, the DMT symbol isdefined as the DMT symbol in the R-FEXT duration. In the other cases,the DMT symbol is defined as a DMT symbol in an R-NEXT duration. Thedefined durations include the round trip delay mentioned before and asystem margin.

When the first DMT symbol is synchronized with the head of one cycle ofthe reference signal of 400 Hz, the determination of whether n-th symbolbelongs to the R-FEXT duration or the R-NEXT duration can be performedas follows.

It is assumed that there are 2760 samples in one cycle of the referenceclock signal of 400 Hz, as shown in (1) of FIG. 11. Also, each symbol isassumed to have 272 samples during communication, as shown in (3) ofFIG. 11. Then a parameter S is defined as:S={272*(n−1)}mod2760.

If {S<(a−272)} or {S+16>(a+b)} is satisfied, then it is judged that then-th symbol belongs to a FEXT duration (B duration).

If {(a−272)<S} and {S+16<(a+b)} are satisfied, then it is judged thatthe n-th symbol belongs to an R-NEXT duration (A duration).

FIG. 12 is a diagram showing a method for modified sliding windowtransmission system according to an embodiment of the present invention.As shown in. FIG. 12, two DMT symbols X as outside of sliding window andY as inside of sliding window according to two bitmaps are employed. TheDMT symbols x according to the first bitmap is used in the R-NEXTdurations. The DMT symbols Y according to the second bitmap is used inthe R-FEXT durations.

1-32. (canceled)
 33. A digital subscriber line communicating method fortransmitting a Sync Symbol for defining the boundary of a Super Framecomprising 69 DMT symbols, through a telephone line, which can beaffected by a near end cross-talk (NEXT) or a far end cross-talk (FEXT)from ISDN in the former and the latter halves of a TCM of 400 Hz,characterized in that: five Super Frames form one unit whose duration ismade to coincide with an integer multiple of the time duration (2.5 ms)of the TCM of 400 Hz and the fourth Sync Symbol is transmitted as aninverted Sync Symbol, to the FEXT duration of a receiver side in orderto transmit the boundary of the five Super Frames.
 34. A digitalsubscriber line communicating method for transmitting a Sync Symbol fordefining the boundary of a Super Frame comprising 69 DMT symbols,through a telephone line, which can be affected by a near end cross-talk(NEXT) or a far end cross-talk (FEXT) from ISDN in the former and thelatter halves of a TCM of 400 Hz, characterized in that: five SuperFrames form one unit whose duration is made to coincide with an integermultiple of the time duration (2.5 ins) of the TCM of 400 Hz and thefirst Sync Symbol is transmitted as an inverted Sync Symbol, to the FEXTduration of a central office in order to inform the central office ofthe boundary of the five Super Frames.
 35. An xDSL apparatus fortransmitting a Sync Symbol for defining the boundary of a Super Framecomprising 69 DMT symbols through a telephone line, which can beaffected by a near end cross-talk (NEXT) or a far end cross-talk (FEXT)from ISDN in the former and the latter halves of a TCM of 400 Hz,characterized in that: five Super Frames form one unit whose duration ismade to coincide with an integer multiple of the time duration (2.5 ins)of the TCM of 400 Hz and the fourth Sync Symbol is transmitted as aninverted Sync Symbol, to the FEXT duration of a receiver side in orderto transmit the boundary of the five Super Frames.
 36. An xDSL apparatusfor transmitting a Sync Symbol for defining the boundary of a SuperFrame comprising 69 DMT symbols through a telephone line, which can beaffected by a near end cross-talk (NEXT) or a far end cross-talk (FEXT)from ISDN in the former and the latter halves of a TCM of 400 Hz,characterized in that: five Super Frames form one unit whose duration ismade to coincide with an integer multiple of the time duration (2.5 ins)of the TCM of 400 Hz and the first Sync Symbol is transmitted as aninverted Sync Symbol, to the FEXT duration of a central office in orderto inform the central office of the boundary of the five Super Frames.